Generally, as means for improving the power efficiency of an amplifier using a transistor such as a high-output HEMT (High Electron Mobility Transistor), a microwave harmonic processing circuit configured to reflect harmonics is connected to the output terminal of the transistor.
This microwave harmonic processing circuit is configured to control the waveforms of the current and voltage at the output terminal of the transistor by reflecting harmonics to the transistor. This is done by making the impedance at the transistor's output terminal short-circuited for even-order harmonics and open-circuited for odd-order harmonics. This operation is called class-F operation. The class-F operation eliminates overlaps between the instantaneous current and instantaneous voltage of the output of the transistor and thereby reduces the power consumption in the transistor. Accordingly, the power efficiency of the amplifier can be improved.
FIG. 1 shows a plan view of a conventional microwave harmonic processing circuit of Example 1. FIG. 2 shows an equivalent circuit capable of achieving microwave characteristics which is the design goal of the microwave harmonic processing circuit in FIG. 1. The microwave harmonic processing circuit shown in FIG. 1 is formed by using microstrip lines on a single flat plane; an input microwave main line T11, an output microwave main line T12, and four stubs T21 to T24 each having one end open circuited are connected to each other at a single connecting point 70. The equivalent circuit shown in FIG. 2 can be implemented by using the microstrip lines shown in FIG. 1.
In addition, as a conventional technique, there has been known a microwave harmonic processing circuit for a class-F, high-efficiency power amplifier which is shown in FIG. 3 (Patent Document 1). This microwave harmonic processing circuit includes a microwave main line T11, a microwave main line T12, multiple first stubs T2 to T7, and a synthesis compensation stub T*. The length of each of the microwave main lines T11 and T12 is equal to ¼ of a wavelength λ of the fundamental wave in the output of the transistor. A length L of each of the multiple first stubs T2 to T7 is equal to λ/4 m. The magnitude of the admittance of the synthesis compensation stub T* is equal to that of the synthetic input admittance of the multiple first stubs T2 to T7 but has the opposite sign.
According to this configuration, the microwave main line T11 brings the input impedance against the fundamental wave to zero; the multiple first stubs T2 to T7 bring the impedances in harmonics at a point A to zero; and the synthesis compensation stub T* removes influences of the multiple first stubs T2 to T7 on the fundamental wave.
In addition, as a conventional technique, there has been known a high-efficiency amplifier using a microwave harmonic processing circuit shown in FIG. 4 (Patent Document 2). This amplifier includes a harmonic reflection stub 20A which is connected to the output terminal of an amplification transistor 10A and in which input susceptances at multiple frequencies that are integer multiples of a fundamental frequency fo are diffused. The amplifier includes a fundamental wave matching circuit 30 which has one end connected to the output terminal of the amplification transistor 10A in parallel with the harmonic reflection stub 20A and the other end connected to a load circuit, and is capable of impedance matching of the sum of the output admittance of the amplification transistor 10A and the input susceptance of the harmonic reflection stub 20A to the impedance value of the load circuit.
The harmonic reflection stub 20A includes: one stem stub T21A having one end connected to the output terminal of the amplification transistor 10A; and multiple branch stubs T22A and T23A connected in parallel to the other end of the stem stub T21A in a branching manner. By this configuration, an amplifier with a small mounting area can achieve high efficiency, broad band characteristics.
In addition, as a convention technique, the present inventors have disclosed a class-F amplifier in a 5.8 GHz band in an article given in a European Microwave Conference (Non-Patent Document 1).